Natural gas is produced from various geological formations. Natural gas produced from various geological formations typically contains methane, ethane, propane, and heavier hydrocarbons, as well as trace amounts of various other gases such as nitrogen, carbon dioxide, and hydrogen sulfide. The various proportions of methane, ethane, propane, and the heavier hydrocarbons may vary, for example, depending upon the geological formation from which the natural gas is produced.
Natural gas comes from both “conventional” and “unconventional” geological formations. Conventionally-produced natural gas, or “free gas,” is typically produced from formations where gas is trapped in multiple, relatively small, porous zones in various naturally occurring rock formations such as carbonates, sandstones, and siltstones. Conventionally-produced natural gas is generally produced from deep reservoirs and may either be associated with crude oil or be associated with little or no crude oil. Such conventionally-produced natural gas typically comprises from about 70 to 90% methane and from 5 to 10% ethane, with the balance being propane, heavier hydrocarbons, and trace amounts of various other gases (nitrogen, carbon dioxide, and hydrogen sulfide). These gas streams are termed “lean,” meaning that this natural gas typically contains from about 3 to 5 gallons of ethane and heavier hydrocarbons per thousand standard cubic feet of gas (GPM). Such conventionally-produced natural gas streams are generally supplied as a feed gas stream to a natural gas processing plant (e.g., a NGL recovery plant) at a relatively high pressure, typically at about 900 to 1200 psig. Generally, natural gas processing plants (e.g., NGL recovery plants) are configured to process such conventionally-produced gas.
Unconventionally-produced gas is generally produced from formations including coal seams (also known as coal-bed methane, CBM), tight gas sands, geo-pressurized aquifers, and shale gas. These unconventional reservoirs may contain large quantities of natural gas, but are considered more difficult to produce as compared to conventional reservoir rocks. With recent advances in hydraulic fracking and horizontal drilling, these gas streams can be economically recovered. Such advances have triggered a surge in shale gas exploration (e.g., an unconventional natural gas reservoir). In some gas shales, for example, in the upper northwestern regions in the United States, the natural gas produced from such unconventional reservoirs can be very rich, for example, containing about 50 to 70% methane, 10 to 30% ethane with the balance in propane, heavier hydrocarbons, and trace amounts of various other gases (nitrogen, carbon dioxide, and hydrogen sulfide). These rich gas streams contain 8 to 1.2 GPM of ethane and heavier hydrocarbons. Such unconventionally-produced natural gas streams are generally supplied at relatively lower pressures, typically about 400 to 600 psig.
Thus, although various conventional systems and methods are known to separate ethane, propane, and heavier hydrocarbons from various natural gas (e.g., feed gas) streams, there is a need for improved systems and methods for processing a low pressure rich feed gas stream, for example, for recovering propane and heavier hydrocarbons and, optionally, for recovering ethane.